Copyrighted digital products such as game software and movie software are put into the market in the form of optical discs having a copyrighted digital product recorded thereon. One problem that is seriously troubling copyright holders of digital products as well as optical disc manufactures is distribution of unauthorized discs that store illicitly copied game software or movie software. In order to distinguish their original optical discs from unauthorized discs, the manufacturers provide processing on the original discs so as to prove that they are original. Examples of such processing include (1) applying a coating on an optical disc at any given part, (2) intentionally damaging an optical disc at any given part, and (3) embedding a specific data pattern randomly. Original discs processed in any of the above manner are reproduced after being checked by an authorized reproduction device as follows. That is, when instructed to reproduce an optical disc, a reproduction device manufactured by an authorized manufacturer checks whether the optical disc includes a processed part and whether the processed part is in a normal state, thereby judging whether the optical disc is an original disc. Upon judging the optical as original, the reproduction device reproduces the optical disc. Contrary, when the optical disc includes no processed part, or when the processed part is in an abnormal state, the reproduction device does not reproduce the optical disc. In this manner, only optical discs manufactured under authorization are reproduced while avoiding reproduction of unauthorized discs.
However, even for the sake of copyright protection, such a processed part present on an original disc requires users to handle the original optical disc with care so as not to contaminate or damage the processed part. If a user is in possession of a numerous original discs, he is required to handle every one of these discs with care, which is troublesome. In case where an optical disc is no longer reproducible due to a slight contaminant or damage, complaints against the product will be addressed to the optical disc manufacturer, and the confidence in the optical disc manufacturer will be damaged severely. Such an event is undesirable not only for the users but also for the manufacturer at all.